FIGS. 1A and 1B depict conventional magnetic elements 10 and 10′. The conventional magnetic element 10 is a spin valve and includes a conventional antiferromagnetic (AFM) layer 12, a conventional pinned layer 14, a conventional conductive spacer layer 16 and a conventional free layer 18. Other layers (not shown), such as seed or capping layer may also be used. The conventional pinned layer 14 and the conventional free layer 18 are ferromagnetic. Thus, the conventional free layer 18 is depicted as having a changeable magnetization 19. The conventional conductive spacer layer 16 is nonmagnetic. The AFM layer 12 is used to fix, or pin, the magnetization of the pinned layer 14 in a particular direction. The magnetization of the free layer 18 is free to rotate, typically in response to an external magnetic field. The conventional magnetic element 10′ depicted in FIG. 1B is a spin tunneling junction. Portions of the conventional spin tunneling junction 10′ are analogous to the conventional spin valve 10. Thus, the conventional magnetic element 10′ includes an AFM layer 12′, a conventional pinned layer 14′, a conventional insulating barrier layer 16′ and a conventional free layer 18′ having a changeable magnetization 19′. The conventional barrier layer 16′ is thin enough for electrons to tunnel through in a conventional spin tunneling junction 10′.
Depending upon the orientations of the magnetization 19/19′ of the conventional free layer 18/18′ and the conventional pinned layer 14/14′, respectively, the resistance of the conventional magnetic element 10/10′, respectively, changes. When the magnetization 19/19′ of the conventional free layer 18/18′ is parallel to the magnetization of the conventional pinned layer 14/14′, the resistance of the conventional magnetic element 10/10′ is low. When the magnetization 19/19′ of the conventional free layer 18/18′ is antiparallel to the magnetization of the conventional pinned layer 14/14′, the resistance of the conventional magnetic element 10/10′ is high. To sense the resistance of the conventional magnetic element 10/10′, current is driven through the conventional magnetic element 10/10′. Typically in memory applications, current is driven in a CPP (current perpendicular to the plane) configuration, perpendicular to the layers of conventional magnetic element 10/10′ (up or down, as seen in FIG. 1A or 1B).
In order to overcome certain issues associated with magnetic memories having a higher density of memory cells, spin transfer may be utilized to switch the magnetizations 19/19′ of the conventional free layers 10/10′. Spin transfer is described in the context of the conventional magnetic element 10′, but is equally applicable to the conventional magnetic element 10. Current knowledge of spin transfer is described in detail in the following publications: J. C. Slonczewski, “Current-driven Excitation of Magnetic Multilayers,” Journal of Magnetism and Magnetic Materials, vol. 159, p. L1 (1996); L. Berger, “Emission of Spin Waves by a Magnetic Multilayer Traversed by a Current,” Phys. Rev. B, vol. 54, p. 9353 (1996), and F. J. Albert, J. A. Katine and R. A. Buhrman, “Spin-polarized Current Switching of a Co Thin Film Nanomagnet,” Appl. Phys. Lett., vol. 77, No. 23, p. 3809 (2000). Thus, the following description of the spin transfer phenomenon is based upon current knowledge and is not intended to limit the scope of the invention.
When a spin-polarized current traverses a magnetic multilayer stack such as the spin tunneling junction 10′ in a CPP configuration, a portion of the spin angular momentum of electrons incident on a ferromagnetic layer may be transferred to the ferromagnetic layer. In particular, electrons incident on the conventional free layer 18′ may transfer a portion of their spin angular momentum to the conventional free layer 18′. As a result, a spin-polarized current can switch the direction of the magnetization 19′ of the conventional free layer 18′ if the current density is sufficiently high (approximately 107–108 A/cm2) and the conventional free layer 18′ is sufficiently thin, for instance, preferably less than approximately ten nanometers for Co. Spin transfer-based switching of magnetization dominates over other switching mechanisms and becomes observable when the lateral dimensions of the conventional magnetic element 10/10′ are small, in the range of few hundred nanometers. Consequently, spin transfer-based switching is suitable for higher density magnetic memories having smaller magnetic elements 10/10′.
The phenomenon of spin transfer can be used in the CPP configuration as an alternative to or in addition to using an external switching field to switch the direction of magnetization of the conventional free layer 18′ of the conventional spin tunneling junction 10′. For example, the magnetization 19′ of the conventional free layer 18′ can be switched from antiparallel to parallel to the magnetization of the conventional pinned layer 14′. Current is driven from the conventional free layer 18′ to the conventional pinned layer 14′ (conduction electrons traveling from the conventional pinned layer 14′ to the conventional free layer 18′). The majority electrons traveling from the conventional pinned layer 14′ have their spins polarized in the same direction as the magnetization of the conventional pinned layer 14′. These electrons may transfer a sufficient portion of their angular momentum to the conventional free layer 18′ to switch the magnetization 19′ of the conventional free layer 18′ to be parallel to that of the conventional pinned layer 14′. Alternatively, the magnetization of the free layer 18′ can be switched from a direction parallel to antiparallel to the magnetization of the conventional pinned layer 14′. When current is driven from the conventional pinned layer 14′ to the conventional free layer 18′ (conduction electrons traveling in the opposite direction), the majority electrons have their spins polarized in the direction of magnetization of the conventional free layer 18′. These majority electrons are then transmitted through the conventional pinned layer 14′. The minority electrons, however, are reflected from the conventional pinned layer 14′ to return to the conventional free layer 18′ and may transfer a sufficient amount of their angular momentum to switch the magnetization 19′ of the free layer 18′ antiparallel to that of the conventional pinned layer 14′.
Although the spin transfer effect functions as a switching mechanism for the conventional magnetic elements 10 and 10′, complications arise when synthetic antiferromagnetic structures, such as Co/Ru/Co, are used as pinned layers or free layers. In “Reduction of Spin Transfer by Synthetic Antiferromagnets,” arXiv:cond-mat/0401483v1, 24 Jan. 2004, N. C. Emley, et al. observed significant reductions of the magnetoresistance ratio and of the spin transfer effect when they introduced synthetic antiferromagnetic structures into the pinned layers of their pseudo-spin valves. This reduction was attributed to the reduced polarization of the electrons passing through or being reflected from the synthetic antiferromagnetic structures for the common cases where the thicknesses of the sublayers in the synthetic antiferromagnetic structures are much smaller than the corresponding sublayers's spin diffusion lengths.
Accordingly, what is needed is a system and method for providing an improved magnetic element utilizing spin transfer as a switching mechanism. The present invention addresses such a need.